Field of the Invention
The invention relates to a method and a device for calibrating a probe system including an exhaust probe and a control circuit for a motor vehicle.
As environmental awareness is increasing and resulting exhaust gas regulations are becoming increasingly strict, the need to reduce pollutants in exhaust gases of internal combustion engines in motor vehicles is becoming increasingly important. Compliance with the currently valid emission limits for pollutants such as carbon monoxide (CO), nitrogen oxide (NOx) and hydrocarbons (HC) requires selective engine control on the one hand and catalytic post-treatment of the exhaust gases on the other. For both measures it is necessary to get measurement values with exhaust gas probesxe2x80x94for example lambda probes or NOx probes. The term probe will be used below to mean a unit formed of a sensor, a sensor line and a sensor plug.
It is known to use thick-film sensors to measure the concentration of pollutants in the exhaust gas of an internal combustion engine. Such a sensor is described, using the example of an NOx sensor, by N. Kato et al., in the publication xe2x80x9cPerformance of Thick Film NOx Sensor on Diesel and Gasoline Enginesxe2x80x9d Society of Automotive Engineers, publication 970858, 1997. This NOx sensor has two measuring cells and three oxygen pumping cells and implements the following measuring concept: in a first measuring cell to which the gas which is to be measured is fed via a diffusion barrier, a first oxygen concentration is set through the use of a first oxygen ion pump current, no decomposition of NOx taking place. In a second measuring cell, which is connected to the first measuring cell via a diffusion barrier, the oxygen content is reduced further through the use of a second oxygen ion pump current and NOx is decomposed at a measuring electrode. The oxygen which is generated in this way is taken as a measure of the NOx concentration. The entire NOx sensor is heated to a higher temperature, for example 700xc2x0 C., through the use of an electric heating element.
When such sensors are manufactured, large fabrication tolerances occur for technological reasons. In addition, the properties of the sensor, for example the impedance of the pump cells, change over time. Because the signal currents of exhaust probes with such sensors are usually only in the nA range, the component tolerances of the control circuit also influence the accuracy or precision of the measurement to a considerable degree. A measuring precision which is sufficient for a mass produced component in a motor vehicle can thus be ensured only through the use of individual standardization of the probe control circuit systemxe2x80x94referred to below as probe systemxe2x80x94and, in addition, a recalibration must be possible during the service life of the probe.
It is accordingly an object of the invention to provide a method for calibrating a probe system which overcomes the above-mentioned disadvantages of the heretofore-known methods of this general type and which makes it possible to perform an individual calibration on exhaust probes, together with the associated control circuit, at the end of the manufacturing process and in the course of the service life of such exhaust probes, and which allows to compensate the component tolerances of the control circuit.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for calibrating a probe system, the method includes the steps of:
providing a probe system including an exhaust probe and a control circuit with a microcontroller and an analog circuitry, the exhaust probe operating according a principle of a galvanic oxygen concentration cell with a solid electrolyte;
using the microcontroller in conjunction with the analog circuitry for controlling pump currents;
acquiring measured values with the exhaust probe under given test conditions;
reading the measured values into the microcontroller; and
storing the measured values read into the microcontroller in a programmable read-only memory provided in the control circuit as correction values or test values.
According to an alternative embodiment of the invention, there is provided a method for calibrating a probe system, which includes the steps of:
providing a probe system including an exhaust probe and a control circuit with a microcontroller and an analog circuitry, the exhaust probe operating according a principle
of a galvanic oxygen concentration cell with a solid electrolyte;
using the microcontroller in conjunction with the analog circuitry for controlling pump currents;
acquiring measured values with the exhaust probe under given test conditions;
reading the measured values into the microcontroller;
evaluating the measured values in the microcontroller for providing evaluated measured values; and
storing the evaluated measured values in a programmable read-only memory provided in the control circuit as correction values or test values.
According to another mode of the invention, the correction values or test values are stored in characteristic maps in the programmable read-only memory.
According to yet another mode of the invention, the programmable read-only memory is provided as an integrated programmable read-only memory integrated into the microcontroller.
According to a further mode of the invention, the measured values of the exhaust probe are acquired under electrically neutral conditions, and the measured values recorded under the electrically neutral conditions are stored as the correction values in the programmable read-only memory.
According to another mode of the invention, the pump currents
are corrected with the correction values.
According to yet another mode of the invention, the test values are used as a reference for a subsequent recalibration of the probe system.
With the objects of the invention in view there is also provided, a device for calibrating a sensor system, including:
an exhaust probe including a test chamber to be provided with a test gas with variable test parameters;
a control circuit connected to the exhaust probe;
a test control computer for setting and monitoring the variable test parameters in the test chamber; and
a data line, the control circuit and the test control computer communicating with one another via the data line.
According to another feature of the invention, the control circuit has a serial interface, and the serial interface connects the data line to the control circuit.
According to yet another feature of the invention, the exhaust probe is a motor vehicle exhaust probe.
According to another feature of the invention, the exhaust probe operates according a principle of a galvanic oxygen concentration cell with a solid electrolyte, and the exhaust probe acquires measured values under given test conditions, and the control circuit controls pump currents for the exhaust probe, and the control circuit stores the measured values as correction values or test values.
According to a further feature of the invention, the control circuit corrects the pump currents based on the measured values.
If a microcontroller in conjunction with analog circuitry is used to control the pump currents, it is possible to store application-specific data of the probe system. The measured values which are recorded under predefined test conditions are stored as correction or test values in a programmable read-only memory (ROM), for example an EPROM (erasable programmable read-only memory), which is preferably integrated into the microcontroller. The test values are then used as a reference for independent recalibration of the probe system during its service life. The correction values are used to compensate further component tolerances, for example offset voltages of operational amplifiers.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and device for calibrating a probe system, composed of an exhaust probe and a control circuit for a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.